1. Field of the Invention
The present invention relates to infrared pass filters and pixel sensors, and more particularly, to infrared pass filters and infrared signal correction in an image.
2. Description of Related Art
Imaging sensors or devices based on silicon technology typically require the use of an infrared blocking element somewhere in the optical chain. The purpose of this element is to prevent infrared (IR) energy (typically considered to be light with a wavelength longer than 780 nm) from entering the imaging array. Silicon-based devices will typically be sensitive to light with wavelengths up to approximately 1200 nm. If IR is permitted to enter the array, the device will respond and generate an output image signal based on the detected IR. Since the purpose of an imaging system in the context of consumer photography is to create a representation of the visible light present in a scene, the unwanted IR will introduce a false response and distort the image. In a monochrome (black and white) imaging system, the result can be an obviously distorted rendition. For example, foliage and human skin tones may appear unusually light as they typically radiate more IR energy than their surroundings. In a color imaging system, the introduction of IR will distort the coloration and produce an image with incorrect and de-saturated color.
A common method for preventing IR from reaching the imaging array is to use either ionically colored glass or a thin-film coating on glass to create an optical element which passes only visible light (typically in the range of 380 nm to 780 nm) and blocks IR. This element can be placed in front of the taking lens, located within the lens system, or it can be incorporated into the imager package. The principle disadvantages to this approach are cost and added system complexity. The cost of ionically colored glass element can be as high as approximately $1.50 to $2.00 in volume. Thin film coatings can be implemented at a somewhat lower cost at approximately $0.50 to $1.00 in volume, but suffer from the additional disadvantage of exhibiting a spectral shift as a function of angle. Thus, in an imaging system, thin film coatings do not provide a uniform transmittance characteristic from the center of the image to the edge of the image. Both filter types add to system complexity and cost by introducing an extra piece-part which must be assembled into the imaging system and tested. In addition, both filter types reduce the amount of visible light energy that reaches to the imaging array.
Traditional IR filtering techniques also completely eliminate the detection IR energy. Therefore, these techniques do not work for applications which require the measurement of IR energy to capture an IR xe2x80x9cimagexe2x80x9d. For example, these techniques could not be used for imaging applications such as night vision image sensors or mixed mode sensors where IR energy needs to be detected along with visible light energy.
It can therefore be desirable to provide for an imaging system which does not use IR blocking filters, but in which IR does not appreciably affect resulting visible image signals.
In one embodiment, a method is provided having the first step of generating a first signal representing a first amount of light detected by a first pixel sensor, where the first amount of light is composed of light in a first spectrum and in a second spectrum. The method continues with generating a second signal representing a second amount of light detected by a second pixel sensor, where the second amount of light is composed of light in the second spectrum. Then, the method subtracts the second signal from the first signal. The present invention also provides a system for performing the above method.